1. Field of the Invention
The invention relates to a reflective screen for projectors, and more particularly to a reflective screen capable of increasing image contrast.
2. Description of the Related Art
A front projection display, referred to as a projector, typically uses a reflective screen to display images. Image signals projected on and reflected by the reflective screen are seen by viewers as images. Conventional reflective screens typically comprise flexible material, thus, they are easily rolled up for storage. Reflective screens, however, have same drawbacks, such as reduced image contrast due to reflected environmental light.
Recent improvements in reflective screens include increased image contrast in bright environmental light, increasing flexibility and durability, and reduced fabrication costs.
U.S. Pat. No. 5,335,022 discloses a screen with a high image contrast. The screen selects polarized optical signals from a projector but not environmental light to increase the image contrast. A polarizing filter layer is adhered to the screen, thus, passage of optical signals from the projector and the optical signals parallel to the polarizing filter layer is allowed. Polarized environmental light vertical to the polarizing filter layer is, however, filtered. Environmental light must be polarized prior to increasing the image contrast of the screen, thus, the screen is less workable.
U.S. Pat. No. 5,625,489 discloses a combination screen comprising one reflective layer and three polarization controlling layers. The polarization controlling layer has biconvex lens disposed between a photosensitive polarization film and a ¼ wave plate. Since optical signals from a projector are focused and reflected through the lens, the photosensitive polarization film can be activated to allow passage of light. When incident, half of the environmental light is blocked by the photosensitive polarization film. The other half of the light is reflected from the reflective layer after passing through the ¼ wave plate, and then passes again through the ¼ wave plate. Thus, the photosensitive polarization film blocks the other half of the light. The screen of this invention is relatively less flexible.
U.S. Pat. No. 6,987,610 discloses a screen for front projection displays sequentially comprising a substrate, an absorbing material and an uppermost luminescent material. Image signals from a projector excite the luminescent material simultaneously emitting image signals from the projector. The absorbing material absorbs all environmental light except for light of three primary colors (red, green and blue), thus the image contrast of the screen increases.
US patent publication No. 2004/0150883 discloses a screen for a projector having high image contrast and excellent flexibility. Fabrication of the screen is inexpensive and easily to be carried. As shown in FIG. 1A, the screen includes a substrate 20 capable of absorbing light, an optical film 21, a diffusion layer 22 and a protection layer 23. The optical layer 21 comprises layers of high refractive index material H1-H51 and low refractive index material L1-L51. The optical layer 21 does not absorb light, and intensely reflects the light of three-primary colors. Since image signals from the projector consist of three-primary colors, the screen is able to reflect the image signals. FIG. 1B shows a shift of the spectral reflectance of the optical film when the incident angles of light are 0° and 30°.
US patent publication No. 2004/0240053 discloses a reflective screen having an optical film. The screen is made of polymer material with solvent. As shown in FIG. 2A, the screen comprises a light absorption layer 34, a transparent substrate 30, a stacked optical film 31, an angle calibrating layer 33 and a diffusion layer 34. The stacked optical film 31 comprises alternate layers of high refractive index material 1H-9H and low refractive index material 1L-9L. FIG. 2B shows a reflection spectrum of the optical film when the incident angles of light are 0° and 30°.
Although the above-mentioned conventional reflective screens may increase image and color contrast, they still suffer from some drawbacks in system and structure. A novel reflective screen capable of increasing image contrast in bright environmental light with lower fabrication cost is thus desirable.